Optical scanning systems are enjoying ever-growing utility in a variety of products and for a diversity of applications. For the purpose of understanding the subject invention "image or optical scanners" are defined as systems including one or more photosensor arrays operatively disposed to scan a pattern of information or data and generate electrical signals corresponding thereto.
Optical scanners may be readily adapted to address a wide variety of data inputs. The data may be a pattern of graphic information in the form of a photograph, a drawing, a design on fabric or the like, or the information being scanned may be alpha-numeric data such as printed or written matter. In other instances, the information may be a three dimensional pattern, such as the surface of a solid. Basically, optical scanners convert a pattern of data into electrical signals which may be supplied to downstream apparatus for further processing, storage or display. Image scanners are incorporated into facsimile machines, copying machines, computer input terminals, CAD-CAM systems and the like. Additionally, image scanners are utilized in production processes to inspect surfaces of materials such as plywood, fabric, plastic's textile, and metal. The typical image scanner includes one or more photoresponsive circuits configured and disposed so as to either simultaneously or sequentially address various portions of the surface being scanned.
Currently, there are several basic approaches currently pursuant to which image scanners operate. Charge coupled devices (CCDs) form the basis for one such approach. CCD's are solid state devices, typically formed from single crystal silicon and include therein an array of photosensor elements. CCDs have a high degree of photosensitivity and are capable of providing high resolution. However, CCDs are relatively small in size; the typical CCD array is a two dimensional matrix approximately one centimeter square, and the largest CCDs currently produced are one dimensional arrays no greater than approximately 3 to 4 inches in length. These size contraints impose restrictions on the utility of CCDs in scanners. In those instances where a pattern of information having dimensions larger than that of the CCD is being scanned, an optical system must be utilized to project that pattern of information at a reduced size onto the surface of the CCD. Such optical systems will effectively reduce the resolution of the CCD.
In addition to the loss of resolution introduced by the optical system itself, the actual reduction process degrades the effective resolution of the pattern of information being sensed. For example, a typical two dimensional CCD array is one centimeter square and includes therein 256,000 photosensor units, generally referred to as pixels. To translate this into photographic terminology, the equivalent resolution would be about 50 lines/mm for the one centimeter square CCD array. If a pattern of information occupying an area of 35.times.35 millimeters were projected down onto this one centimeter square charge coupled device, the effective resolution of the 35 millimeter square image would fall to approximately 15 lines/mm. For the sake of comparison, medium resolution photographic film is generally capable of resolving approximately 120 lines/mm. Efforts to improve resolution using single crystal integrated circuits encounter at least two significant problems. The first is that integrated circuit chips formed on single crystal silicon wafers must be as small as possible to enable acceptable yields and to meet requirements of economical manufacturing. The second problem, related to the first, is that in order to increase the packing density in the small available chip area, finer and finer photolithography must be used with resulting increases in the cost of manufacture. For these reasons, among others, high resolution sensing (high resolution being defined relative to photographic capabilities) of patterns of information of practical size with present CCD technology is not economically achievable.
With improvements in lithographic techniques it is anticipated that one centimeter square CCDs may ultimately be fabricated to include 1.4 million pixels therein. This translates to a resolution of approximately 120 lines/mm on the one centimeter square device and a corresponding effective resolution of 34 lines/mm for a 35 millimeter square pattern of information projected thereonto. The only way the resolution of the CCD could be further increased is by increasing either the density of pixels in the CCD or the size of the device itself. Both approaches present significant problems. On one hand, the difraction limit of light will ultimately impose limits on any photolithographic process utilized to pattern CCDs although constraints of practicality and cost will generally intervene first to set the limit on pixel density. On the other hand, processing constraints will limit the size of crystalline CCDs that can be manufactured. Single crystal wafers cannot generally be economically manufactured in sizes exceeding perhaps six to eight inches in diameter. Furthermore, processing steps can introduce defects into such devices. Increasing the size of a crystalline device, especially while maintaining strict limits on the size of the photolithographic features thereof imposes a great burden of cost insofar as the likelihood of creating defects exponentially increases along with a dramatic decrease in the yield of devices. The result is that the cost of finished product increases exponentially with increasing device size. It will thus be appreciated that even utilizing the most optimistically projected pixel densities and single crystalline CCD sizes, high resolution optical scanners adapted to sense a pattern of information in an area exceeding several square inches cannot be economically manufactured utilizing such technology.
Deposited thin film devices represent another approach to the fabrication of optical scanners. Thin film devices may be economically manufactured over large areas by the vapor deposition of layers of appropriate semiconductor materials onto a variety of substrates. By appropriately patterning these layers, for example, through the use of presently available photolithographic techniques, a variety of device configurations may be provided.
Recently, considerable progress has been made in developing processes for depositing thin film semiconductor materials. Such materials can be deposited to cover relatively large areas and can be doped to form p-type and n-type semiconductor materials for the production of semiconductor devices such as p-i-n type photodiodes equivalent, and in some cases superior to those produced by their crystalline counterparts. One particularly promising group of thin film materials are the amorphous materials. As used herein, the term "amorphous" includes all materials or alloys which have long range disorder although they may have short or intermediate range order, or even contain at times, crystalline inclusions. Also as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur.
It is now possible to prepare by glow discharge, or other vapor deposition processes, thin film amorphous silicon, germanium or silicon-germanium alloys in large areas, said alloys possessing low concentrations of localized states in the energy gap thereof and high quality electronic properties. Techniques for the preparation of such alloys are fully described in U.S. Pat. Nos. 4,226,898 and 4,217,374 of Stanford R. Ovshinsky, et al., both of which are entitled "Amorphous Semiconductor Equivalent to Crystalline Semiconductors" and in U.S. Pat. Nos. 4,504,518 and 4,517,223 of Stanford R. Ovshinsky, et al., both of which are entitled "Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy"; the disclosures of all of the foregoing patents are incorporated herein by reference.